Compound semiconductor device and its manufacture method

ABSTRACT

A vertical type GaN series field effect transistor having excellent pinch-off characteristics is provided. A compound semiconductor device includes a conductive semiconductor substrate, a drain electrode formed on a bottom surface of the conductive semiconductor substrate, a current blocking layer formed on a top surface of the conductive semiconductor substrate, made of high resistance compound semiconductor or insulator, and having openings, an active layer of compound semiconductor burying the openings and extending on an upper surface of the current blocking layer, a gate electrode formed above the openings and above the active layer, and a source electrode formed laterally spaced from the gate electrode and formed above the active layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/548,622, filed on Aug. 27, 2009, which is a Continuation Applicationof International Application No. PCT/JP2007/053689 filed on Feb. 27,2007, the entire contents of which being incorporated herein byreference.

FIELD

The embodiments discussed herein are related to a compound semiconductordevice and its manufacture method, and more particularly to a compoundsemiconductor device having a vertical current path and its manufacturemethod.

GaN series (compound semiconductor) indicates Al_(x)In_(y)Ga_(1-x-y)N(0≦x≦1.0, 0≦y≦1.0).

BACKGROUND

Vigorous developments are made for compound semiconductor devices usingGaN or GaN series compound semiconductor. GaN has a wide or high bandgap of 3.4 eV, and enables a high breakdown voltage operation. Varioussemiconductor devices can be manufactured using GaN series compoundsemiconductor.

GaN has a high breakdown voltage, and is expected to be applied to usagerequiring high voltage and high speed operation, such as a high electronmobility transistor (HEMT) for a mobile phone base station. There arevarious reports on GaN-HEMT formed by growing crystals of GaN/AlGaN on asubstrate such as sapphire, SiC, GaN and Si and using a GaN layer as anelectron transport or channel layer. A value over 300 V is currentlyreported as a breakdown voltage in an off current state. Transistorsutilizing the field effect including HEMT and MESFET may collectivelycalled field effect transistors in some cases.

Metal organic chemical vapor deposition (MOCVD) is mainly used as acrystal growth method. A method called hydride vapor phase epitaxy(H-VPE) is being studied in recent years, which method produces metalchloride by reacting HCl with group III metal and further conductingreaction with ammonia and the like to grow nitride semiconductor.

If a GaN series field effect transistor of a lateral structure disposinga source, a channel and a drain along a substrate surface layer is usedas a power device for vehicles or the like, it is difficult to increasea current density per area so that a chip size becomes large. In orderto increase a current density per area, it is preferable to use avertical structure in which carriers move along a substrate thicknessdirection.

JP-A-2003-051508, JP-A-2004-31896, JP-A-2005-203753, and WO2003/071607,which are incorporated herein by reference, propose a vertical structurethat a source is disposed on the top surface of a semiconductorsubstrate, a drain is disposed on the bottom surface, and a gate isdisposed on a vertical side wall formed by vertically etching thesubstrate from the top surface to an intermediate depth. Since atransistor structure is formed along the depth direction of thesubstrate, it is expected to increase an area utilization efficiency.With this gate electrode structure formed on a vertical side wall formedby vertically etching the semiconductor layer, processes becomecomplicated, and there is a high possibility of many issues such asprocess precision and yield.

JP-A-2002-16262, which is incorporated herein by reference, proposes astructure that a source electrode and a gate electrode of interdigitalshape are formed on the top surface of a substrate, and a drainelectrode is formed on the bottom surface. An n⁺-type GaN source regionis surrounded by a p-type well and an i-type well, and an n-type GaNchannel region is formed under the gate electrode traversing the n-typewell and the i-type well. Under the n-type channel region, an n⁻-typeGaN layer, an n⁺-type GaN layer and an n⁺⁺-type Si substrate aredisposed. The electrodes are formed on flat surfaces. In order toelectrically separate the source region from the substrate bottom side,a recess is formed by etching, and epitaxial growth on the recess isperformed for growing the i-type well, the p-type well and the n⁺-typesource region. Further a recess is formed by etching, and epitaxialgrowth on the recess is performed for growing the channel region. It maybecome necessary to planarize the epitaxial growth surface. It can beconsidered that manufacture processes are not simple.

FIG. 5A schematically illustrates the structure of a vertical typeGaN-HEMT reported by University of California, Santa Barbara (ConferenceDigest of Device Research Conference, 2002, pp. 31-32). An n-type GaNlayer 102 is grown on a single crystal sapphire substrate 101, and ap-type GaN layer 103 serving as a current blocking layer 103 is formedon the n-type GaN layer 102. The p-type GaN layer 103 is etched to formopening in current flowing region. An n-type GaN layer 105 is grown tofully bury the opening of the current flowing region, and a non-dopedGaN active layer 106 serving as an electron transport layer is formed onthe p-type GaN layer 103 and the n-type GaN layer 105. An n-type AlGaNlayer 108 serving as an electron supply layer is grown above thenon-doped GaN layer 106 via an non-doped AlGaN layer 107 serving as aspacer layer. A deep potential well is formed at the interface of thenon-dopes GaN layer contacting the AlGaN layer having a wide band gap,so that two dimensional electron gas 2DEG is generated. An n-type GaNlayer 109 is grown on the n-type AlGaN layer 108. Isolation is performedby recess etching. A silicon nitride layer 110 is formed on the surfaceby plasma CVD. The silicon nitride layer on electrode forming areas areremoved, and a gate electrode G in Schottky contact and a sourceelectrode S in ohmic contact are formed thereon. The layers down to then-type GaN layer 102 are etched, and a drain electrode D is formed onthe n-type GaN layer 102.

FIG. 5B illustrates the characteristics of the GaN-HEMT thus formed.Drain current Ids does not become zero even in an off-state, i.e.insufficient pinch-off characteristics.

SUMMARY

According to one aspect of the invention, there is provided a compoundsemiconductor device including:

a conductive semiconductor substrate;

a drain electrode formed on a bottom surface of the conductivesemiconductor substrate;

a current blocking layer formed on a top surface of the conductivesemiconductor substrate, made of high resistance compound semiconductoror insulator, and having openings;

an active layer of compound semiconductor burying the openings andextending on an upper surface of the current blocking layer;

a gate electrode formed above the opening and above the active layer;and

a source electrode laterally spaced from the gate electrode and formedabove the active layer.

According to another aspect of the invention, there is provided a methodfor manufacturing a compound semiconductor device comprising:

(a) forming a current blocking layer on a top surface of a conductivesemiconductor substrate, the current blocking layer being made of highresistance compound semiconductor or insulator, and having openings;

(b) forming an active layer of compound semiconductor burying theopenings and extending on an upper surface of the current blockinglayer;

(c) forming a gate electrode above the opening and above the activelayer;

(d) forming a source electrode laterally spaced from the gate electrodeand above the active layer; and

(e) forming a drain electrode on a bottom surface of the conductivesemiconductor substrate.

The high resistance or insulating current blocking layer can enhancecurrent blocking characteristics.

High current blocking effect can be established by using a potentialbarrier based on the hetero junction.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are a schematic cross sectional view and a schematicplan view of a GaN series vertical type HEMT according to a firstembodiment of the invention. FIG. 1C is a sketch illustrating anepitaxially grown layer according to a modification.

FIGS. 2A and 2B are schematic cross sectional views of a hydride VPEsystem and an MOCVD system.

FIGS. 3A, 3B, 3C and 3D are a schematic cross sectional viewillustrating the structure of a GaN series vertical type HEMT accordingto a second embodiment of the invention, a graph illustrating carrierconcentration profiles in depth direction, a graph illustrating acarrier concentration profile in lateral direction in opening portion,and a graph illustrating I-V characteristics.

FIGS. 4A and 4B are a schematic cross sectional view illustrating thestructure of a GaN series vertical type HEMT according to a thirdembodiment of the invention, and a graph illustrating I-Vcharacteristics.

FIGS. 5A and 5B are a cross sectional view illustrating the structure ofa GaN series vertical type HEMT according to prior art, and a graphillustrating I-V characteristics thereof.

DESCRIPTION OF EMBODIMENTS

Studies have been conducted for investigating the cause of leak currentof GaN series HEMT illustrated in FIG. 5A. The p-type GaN layer 103formed above the single crystal sapphire substrate 101 via the GaN layer102 constitutes a current blocking layer. Since the sapphire substrateand GaN have different lattice constants, crystal dislocation occurs sothat excellent crystallinity cannot be expected. It is thereforeconsidered that the current blocking function of the p-type GaN layer103 is imperfect and electrons may move along dislocations through thep-type GaN layer 103. Although the two dimensional electron gas 2DEG inthe non-doped GaN layer 106 near at the interface with the non-dopedAlGaN layer 107 can be controlled by the gate electrode G, it isdifficult for the gate electrode G to control electrons which have moveddownwards from the two dimensional electron gas 2DEG. It is desired tosuppress motion of electrons downwards from the two dimensional electrongas 2DEG and to enhance the current blocking function of the currentblocking layer.

AlN has a larger band gap than that of GaN so that AlN can form apotential barrier for carriers in GaN. It is easy to form an AlN regionhaving a high resistivity. A GaN region contacting the AlN regionbecomes more p-type and forms a potential barrier for electrons.

FIG. 1A is a schematic cross sectional view illustrating the structureof a GaN series vertical type HEMT according to the first embodiment ofthe invention. An AlN layer 2 is grown on an n-type single crystalc-plane SiC substrate 1 to a thickness of about 3 μm by hydride (H—)vapor phase epitaxy (VPE).

FIG. 2A is a schematic cross sectional view illustrating the structureof a hydride VPE system. A high frequency coil 31 for induction heatingis wound around a quartz reaction tube 30, and a carbon susceptor 32 forplacing thereon a substrate 1 is disposed in the quartz reaction tube.Two gas introducing pipes 34 and 35 are coupled to the upstream end ofthe reaction tube 30 illustrated left in FIG. 2A, and a gas exhaust pipe36 is coupled to the downstream end of the reaction tube 30. A boat 38is disposed in the reaction tube 30 at an upstream position of thesusceptor 32, and a source 39 of a group III element of compound to begrown is accommodated in the boat. For example, the source 39 is Al ifAlN is to be grown, and Ga if GaN is to be grown. Ammonia NH₃ gas isintroduced as N source gas via the gas introducing pipe 34, and HCl isintroduced via the gas introducing pipe 35. HCl reacts with the groupIII element source 39 in the boat 38 to produce group III elementchloride AlCl. Source gases AlCl and NH₃ are transported to thesubstrate and are allowed to react on the substrate surface to grow AlN.Surplus gas is exhausted via the gas exhaust pipe 36 to a detoxificationtower.

An AlN layer 2 is grown to a thickness of about 3 μm on a (0001) plane(c-plane) single crystal conductive SiC substrate 1 by H-VPE. The groupIII element source 39 in the boat 38 of the H-VPE system is Al. Theconditions of H-VPE are set, for example, as follows:

pressure: normal pressure,

gas flow rate: HCl: 100 sccm,

-   -   NH₃: 10 LM (litter per minute), and

temperature: 1100° C.

A resistivity of the AlN layer 2 can be set extremely high, e.g., higherthan 1×10⁵ (represented as 1E5) Ωcm.

A resist mask is formed on the AlN layer 2, and the AlN layer is etchedto form openings (each having a size 0.5 μm×500 μm) in current pathregions. The resist mask is removed thereafter. Above the SiC substrate1 and AlN layer 2, a HEMT lamination structure is grown by metal organicchemical vapor deposition (MOCVD).

FIG. 2B is a schematic cross sectional view illustrating the structureof an MOCVD system. A high frequency coil 41 is wound around a quartzreaction tube 40, and a carbon susceptor 42 for placing thereon asubstrate 1 is disposed in the reaction tube 40. Two gas introducingpipes 44 and 45 are coupled to the reaction tube 40 at the upstream endto supply source gases for compound. For example, NH₃ as N source gas isintroduced via the gas introducing pipe 44, and organic group IIIelement compound source material such as trimethylaluminum,trimethylgallium and trimethylindium as group III element source gas isintroduced via the gas introducing pipe 45. Crystal growth progresses onthe substrate 1, and surplus gas is exhausted via the gas exhaust pipe46 coupled to the reaction tube at a downstream end to a detoxificationtower. If MOCVD is to be performed in a low pressure atmosphere, the gasexhaust pipe 46 is coupled to a vacuum pump, and an exhaust port of thevacuum pump is coupled to a detoxification tower.

The conditions of MOCVD are set, for example, as in the following:source materials and flow rates:

trimethylgallium (TMG): 0 to 50 sccm,

trimethylaluminum (TMA): 0 to 50 sccm,

trimethylindium (TMI): 0 to 50 sccm, and

ammonia (NH₃): 20 SLM,

n-type impurity: silane (SiH₄),p-type impurity: biscyclopenta-dienyl-magnesium (Cp2Mg),pressure: 100 torr, andtemperature: 1100° C.

First, an n-type GaN layer 5 is grown to a thickness of about 3 μm.Since GaN is hard to grow on SiC, an AlGaN layer containing Al at 10 at% or more is grown at an initial growth stage. The n-type GaN layer 5 isthereafter grown. Lateral growth of GaN is much faster than verticalgrowth. The n-type GaN grows on a side wall of the c-plane AlN layer ata growth speed five times or more of that on the top surface of thec-plane AlN layer. Thus, the n-type GaN layer 5 preferentially buriesthe openings of the AlN layer 2, and after the openings are buried, stepof grown regions above the openings and the top surface of the AlN layer2 is gradually extinguished by lateral growth.

Surface of the AlN layer is oxidized to some extent during patterning byetching. Oxygen is removed through reaction with ammonia gas. then, itbecomes possible to grow GaN. Vertical growth speed remains slower thanthe lateral growth speed on the side wall.

A non-doped GaN layer 6 is grown to a thickness of about 0.2 μm on then-type GaN layer 5 doped with Si at 4×10¹⁸ (represented as 4E18) cm⁻³.The non-doped GaN layer 6 is an active layer serving as an electrontransport or channel layer. The GaN layer contacting the flat AlN layer2 becomes more p-type at its interface due to the piezo effect. Thep-type tendency or conductivity raises the potential of the GaN layernear at the interface, and the potential lowers as the position becomesremoter (upper) from the interface. This potential gradient has adirection of expelling or repelling electrons.

A non-doped AlGaN layer 7 is grown to a thickness of about 5 nm on theGaN layer 6, and then an n-type AlGaN layer 8 doped with Si at 4E18 cm⁻³is grown to a thickness of about 20 nm. The n-type AlGaN layer 8 servesas an electron supply layer for supplying electrons to the active layer6, and the non-doped AlGaN layer 7 serves as a spacer layer forseparating the electron supply layer from the active layer. A deeppotential well is formed at the interface of the non-doped GaN layer 6contacting the AlGaN layer 7 having a wide band gap, and two dimensionalelectron gas 2DEG is generated thereat.

An n-type GaN layer 9 doped with Si at 5E18 cm⁻³ is grown to a thicknessof about 7 nm on the n-type AlGaN layer 8. An isolation region is formedby recess etching. A silicon nitride (SiN) film 11 is deposited on thesurface by plasma CVD. A current collapse phenomenon which changes anon-resistance during operation can be avoided by forming the GaNprotective layer 9 and SiN layer 11 on and above the n-type AlGaNelectron supply layer 8. A resist mask is patterned, and the SiN film isselectively etched and removed with SF₆, and then the GaN protectivelayer 9 is etched and removed with chlorine gas by time-controlledetching, in the ohmic contact region. A Ta film 14 and an Al film 15 onthe Ta film are laminated to form a source electrode S.

Similarly, a drain electrode D is formed on the bottom surface of theSiC substrate 1. The SiN film 11 in gate contact region is removed. Aresist mask pattern is formed, and an Ni film 12 and an Au film 13 onthe Ni film are laminated on the n-type GaN layer 9, to form a gateelectrode G by lift-off.

FIG. 1B is a schematic plan view illustrating the layout of the sourceelectrode S and gate electrode G. The stripe-shaped current pass regionsare indicated by broken lines. The comb-shape gate electrode G isdisposed having teeth covering the current path regions, and thecomb-shape source electrode S is disposed having teeth on both sides ofeach tooth of the gate electrode G. Carriers are supplied from thesource electrode portions on both sides of each current pass region,controlled by the gate electrode, and reach the substrate and drainelectrode via the current path regions. FIG. 1A is a cross sectionalview illustrating a portion of one current pass region and the sourceelectrodes S on both sides of the current pass region. Isolation regionis formed outside the element region 10 indicated by a broken line.

According to the embodiment, the interface of the upper GaN layercontacting the flat AlN layer becomes more p-type due to the piezoeffect. This p-type tendency raises the potential and suppressesdownward motion of electrons from the two dimensional electron gas(2DEG). Further, the AlN layer having a wide band gap blocks pass ofelectrons. For example, even if electrons enter the AlN layer, the AlNlayer itself has a very high resistance. A GaN series vertical type HEMTcan be formed having excellent pinch-off current characteristics in anoff state. Since the SiC substrate has a high thermal conductivity, asemiconductor device having good heat radiation characteristics can beprovided.

Although the AlN current blocking layer is made of the AlN layer havinga thickness of about 3 μm, the AlN layer may be made more thicker. SinceH-VPE can provide a fast growth speed, an AlN layer having a thicknessof, e.g., 10 μm can be grown easily.

FIG. 1C is a sketch drawn from a TEM photograph taken after an AlN layer2 is grown to a thickness of about 12 μm on an SiC substrate 1 by H-VPEand a GaN layer 3 is grown on the AlN layer.

Since H-VPE growth also has a function of reducing crystal dislocations,when a thick AlN layer is grown, its crystallinity is expected to beimproved. Since an H-VPE grown layer contains Cl, there is a possibilitythat the electrode contact characteristics may be adversely affected.When an AlN layer, an AlGaN layer, an AlGaInV layer or the like is grownby MOCVD on an AlN layer grown by H-VPE, the MOCVD grown layer can serveas a CI diffusion blocking layer. Here, an AlN current blocking layermay also be grown by MOCVD. In the first embodiment, the openings formedthrough the AlN current blocking layer are buried with the n-type GaNlayer. Other structures are also possible.

FIG. 3A is a schematic cross sectional view illustrating the structureof a GaN series vertical type HEMT according to the second embodiment ofthe invention. Similar to the first embodiment, an AlN layer 2 is grownto a thickness of about 3 μm on an n-type single crystal SiC substrateby H-VPE, and openings are formed through the AlN layer in current pathregions. An n-type AlGaN layer 3 doped with Si at 1E19 cm⁻³ is grown toa thickness of about 20 nm, above the substrate covering the patternedAlN layer 2, and an n-type GaN layer 4 doped with Si at 1E18 cm⁻³ isgrown to a thickness of about 20 nm on the n-type AlGaN layer, by MOCVD.The openings of the AlN layer 2 are not still buried. A non-doped GaNlayer 6 is grown on the n-type GaN layer 4 to a thickness of about 0.2μm above the AlN layer 2, to bury the openings. The non-doped GaN layer6 is an active layer serving as an electron transport or channel layer.Thereafter, similar to the first embodiment, after a non-doped AlGaNlayer 7 is grown to a thickness of about 5 nm on the GaN layer 6, ann-type AlGaN layer 8 doped with Si at 4E18 cm⁻³ is grown to a thicknessof about 20 nm. An n-type GaN layer 9 doped with Si at 5E18 cm⁻³ isgrown to a thickness of about 7 nm on the n-type AlGaN layer 8.Isolation region is formed by recess etching. A silicon nitride (SiN)film 11 is deposited on the surface by plasma CVD. Similar to the firstembodiment, a source electrode S, a gate electrode G and a drainelectrode D are formed.

As the n-type AlGaN layer 3 is grown on the patterned AlN layer 2 byMOCVD, external diffusion of CI in the AlN layer 2 can be suppressed. Asthe AlGaN layer 3 is grown on the SiC substrate, the GaN layer 4 can begrown on the AlGaN layer 3. As the n-type AlGaN layer 3 and n-type GaNlayer 4 are grown on the top surface and side surfaces of the AlN layer2, the piezo effect occurs at the interface changing from a layercontaining Al to a layer not containing Al (an interface A between then-type AlGaN layer 3 and n-type GaN layer 4) on the upper surfaceportion of AlN. At the same time, the piezo effect occurs also at theinterface between the upper surface of the AlN layer 2 and the AlGaNlayer 3. The GaN layer 4 near at the interface therefore becomes morep-type, its potential is raised and the pinch-off characteristics in anoff state can be improved more than the first embodiment. In contrast,the piezo effect does not occur at the interface between the layercontaining Al and the layer not containing Al (an interface B betweenthe n-type AlGaN layer 3 and n-type GaN layer 4) on the side surface ofthe AlN layer at the openings. This is because the crystal is hexagonal,and bonding of atoms is different between a direction perpendicular tothe substrate surface (c-axis direction) and a direction parallel to thesubstrate surface (a-axis direction). Therefore, on the side surface,only the n-type junction effect occurs and two dimensional electron gasis generated. Therefore, resistance in the vertical direction reduces.On the opening bottom, the n-type AlGaN layer 3 and n-type GaN layer 4are laminated on the SiC substrate 1. Although the piezo effect isexpected to occur essentially, the piezo effect does not apparentlyoccur because of poor flatness of the AlGaN layer on the SiC substrate.Conversely, there occurs island growth on the bottom of the openings, towhich Si impurities may easily invade, the n-type impurity concentrationbecomes high (5E19 cm⁻³) at the bottom portion. The n-type AlGaN layer 3doped with Si at 5E19 cm⁻³ is an n-type conductive layer and allows adrain current to flow vertically at a low resistance.

FIG. 3B is a graph illustrating a carrier concentration in a depthdirection in a center portion #1 of the current pass region under thegate electrode G and in a portion #2 where the AlN current blockinglayer 2 exists. A right hand side region of a broken line is the SiCsubstrate. In the center portion #1 of the current path region, acarrier concentration decreases from the two dimensional electron gas2DEG with the depth, and abruptly increases in the n-type GaN layer 4and n-type AlGaN layer 3 above the SiC substrate at a central portion ofthe opening. In the portion #2 passing through the AlN layer, since then-type GaN layer 4 and n-type AlGaN layer 3 above and on the AlN layer 2become more p-type, carriers will not increase and continue to decreasein the high resistance AlN layer 2.

FIG. 3C illustrates a carrier concentration along a horizontal directionindicated by #3 in FIG. 3A and traversing the AlN current blocking layer2, and the n-type AlGaN layer 3, n-type GaN layer 4 and GaN layer 6 inthe opening. The interface between the n-type GaN layer 4 and n-typeAlGaN layer 3 becomes more n-type and generates two dimensional electrongas. This high mobility two dimensional electron gas provides the effectof decreasing resistance in a vertical direction.

FIG. 3D illustrates I-V characteristics of the GaN series vertical typeHEMT of the second embodiment. In the off state, a drain current ispinched off providing good off characteristics. A resistance in an onstate was improved by 20% from that of the first embodiment.

In the first and second embodiments, the current blocking layer is madeof a high resistance AlN layer. The current blocking layer may be madeof an insulating layer.

FIG. 4A is a schematic cross sectional view illustrating the structureof a GaN series vertical type HEMT according to the third embodiment. AnSiO₂ layer 20 is deposited to a thickness of about 3 μm on a singlecrystal n-type SiC substrate 1 by CVD, and patterned to form currentpass regions. An n-type GaN layer 25 is grown to a thickness of about 3μm by MOCVD on the SiC substrate 1 with the patterned SiO₂ layer 20, byadding Al at 10 at % or more at the initial growth stage. The GaN layer25 is not grown on the insulating layer, but selectively grown in theopenings. When the GaN layer grows exceeding the upper edge of theopening, lateral growth begins. Although the n-type GaN layer 25 may begrown to the level flush with the upper surface of the SiO₂ layer 20,the GaN layer is grown only inside the openings here. A non-doped GaNlayer 26 is grown by MOCVD on the n-type GaN layer 25 and SiO₂ layer 20,to a thickness of about 0.2 μm on the SiO₂ layer 20. Thereafter, similarto the first and second embodiments, a non-doped AlGaN layer 7 is grownto a thickness of about 5 nm on the non-doped GaN layer 26, andthereafter, an n-type AlGaN layer 8 doped with Si at 4E18 cm⁻³ is grownto a thickness of about 20 nm. An n-type GaN layer 9 doped with Si at5E18 cm⁻³ is grown on the n-type AlGaN layer 8 to a thickness of about 7nm. Isolation region is formed by recess etching. A silicon nitride(SiN) film 11 is deposited on the surface by plasma CVD. A sourceelectrode S, a gate electrode G and a drain electrode D are formed.

FIG. 4B illustrates I-V characteristics of the GaN series vertical HEMTof the third embodiment. A drain current slightly flows in an off state.

According to the third embodiment, since the current blocking layer ismade of insulator, leak current passing through the current blockinglayer is extremely small. As compared to the current blocking layer madeof the p-type GaN layer, the current blocking effect can be apparentlyimproved. As compared to the first and second embodiments, the effect ofraising a potential in the lower portion of the non-doped GaN layer 26by the piezo effect is not provided. Therefore, electrons can movedownward from the two dimensional electron gas 2DEG in an off state, andthere is a large possibility that electrons are transported in the GaNlayer near the upper surface of the SiO₂ layer 20 and reach theopenings.

Although the invention has been described in conjunction with theembodiments, the invention is not limited to the embodiments. Forexample, instead of single crystal SiC, single crystal GaN may be usedfor the substrate. The current blocking layer may be made of insulatorother than silicon oxide, such as a silicon nitride and siliconoxynitride. The size, impurity concentration and the like illustrativelydescribed above may be changed in various ways. Instead of HEMT, othervertical type field effect transistors may be formed.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiments of the inventionshave been described in detail, it should be understood that the variouschanges, substitutions, and alterations could be made hereto withoutdeparting from the spirit and scope of the invention.

1. A method for manufacturing a compound semiconductor devicecomprising: (a) forming a current blocking layer on a top surface of aconductive semiconductor substrate, said current blocking layer beingmade of high resistance compound semiconductor or insulator, and havingopenings; (b) forming an active layer of compound semiconductor buryingsaid openings and extending on an upper surface of said current blockinglayer; (c) forming a gate electrode above said openings and above saidactive layer; (d) forming a source electrode laterally spaced from saidgate electrode and above said active layer; and (e) forming a drainelectrode on a bottom surface of said conductive semiconductorsubstrate.
 2. The method for manufacturing a compound semiconductordevice according to claim 1, wherein: said conductive semiconductorsubstrate is made of SiC or GaN; said step (a) includes: (a-1) growingan AlN layer on said conductive semiconductor substrate; and (a-2)patterning said AlN layer to form said openings exposing said conductivesemiconductor substrate, and said step (b) includes: (b-1) growing adoped GaN layer in an area including said openings; and (b-2) growing anon-doped GaN layer on said doped GaN layer.
 3. The method formanufacturing a compound semiconductor device according to claim 2,wherein said step (b) includes (b-3) growing a doped AlGaN layer betweensaid AlN layer and said doped GaN layer.
 4. The method for manufacturinga compound semiconductor device according to claim 3, wherein said step(a-1) grows AlN by H-VPE, and said step (b-3) grows AlGaN by MOCVD. 5.The method for manufacturing a compound semiconductor device accordingto claim 1, wherein said conductive semiconductor substrate is made ofSiC or GaN; said step (a) includes: (a-1) depositing a silicon oxidelayer on said conductive semiconductor substrate; and (a-2) patterningsaid silicon oxide layer to form said openings exposing said conductivesemiconductor substrate, and said step (b) includes: (b-1) growing adoped GaN layer in an area including said openings; and (b-2) growing anon-doped GaN layer on said doped GaN layer.